/Engine/Plugins/Online/OnlineSubsystem/Source/Public/Interfaces/OnlinePartyInterface.h
Returns true if the user specified by MemberId is the leader of the party specified by PartyId
bool IsMemberLeader ( const [FUniqueNetId](API\Runtime\CoreOnline\Online\FUniqueNetId) & LocalUserId, const [FOnlinePartyId](API\Plugins\OnlineSubsystem\FOnlinePartyId) & PartyId, const [FUniqueNetId](API\Runtime\CoreOnline\Online\FUniqueNetId) & MemberId ) const